Shock absorbers and springs are components of a vehicle's suspension system. Together they absorb some of the shock transmitted from the vehicle's wheels and dampen, in advance, the expansion vibration of the springs in order to provide a comfortable ride to the vehicle's occupants.
The running condition of the vehicle changes according to variable factors such as road surfaces, vehicle speeds, steering conditions and the like. As a result, suspension systems having different characteristics are inevitably needed to meet the variable running conditions. In order to provide a suspension system capable of meeting the various running conditions, it is desirable to have a shock absorber capable of properly changing the damping force in relation to varying running conditions of the vehicle.